This invention relates to digital signal recording and reproducing apparatus, and more particularly, to such apparatus which records at a generally constant data rate digital signals that may be supplied at different transmission rates and reproduces those signals without the need for a complex track-following arrangement.
Digital signal recording apparatus have been developed for recording digital signals representative of different types of information. One particular application thereof is to record a video signal in digital form on a moving record medium, such as a magnetic tape, by using a rotary head assembly. In this application, the digital video signals may be of different types, each type exhibiting a different transmission rate, such as an extended play video signal, a normal play video signal having a transmission rate higher than the extended play video signal, and an HDTV video signal having a still higher transmission rate. Notwithstanding these different transmission rates, it is desirable to record such digital video signals at a substantially constant, uniform data rate, thereby simplifying the mechanical assemblies and components, as well as the control electronics used with such recorders. One example of a digital signal recording system operable with video signals exhibiting different transmission rates is described in Japanese laid open patent publication 1-258255.
As described in the aforementioned patent publication, the transmission rates of the extended play, normal play and HDTV signals are in a predetermined ratio, such as the ratio 1:2:4; and the apparatus is operable in an extended play mode, a normal play mode or an HDTV mode to record either the extended play, the normal play or the HDTV signals. To record these different signals, which are selectively supplied, a rotary head assembly comprised of at least two 180.degree. spaced apart subassemblies is used, with each subassembly having two heads displaced from each other in a direction parallel to the pitch of the tracks recorded by those heads. The video signals which are selectively supplied to the rotary head assembly are timebase-compressed to produce recording signals of the same data rate. For example, the extended play digital video signals are timebase compressed to reduce the period thereof by 1/4, the normal play digital video signals are timebase compressed to reduce the period thereof by 1/2 and the HDTV digital video signals are not timebase compressed. This has the effect of increasing the effective transmission rate of the extended play digital video signals by a factor of 4 and increasing the effective transmission rate of the normal play digital video signals by a factor of 2, so that the effective transmission rates of the extended play, normal play and HDTV digital video signals all are equal to the transmission rate of the HDTV signals.
In the aforedescribed Japanese patent publication, the extended play digital video signals are recorded in two tracks substantially simultaneously by one head subassembly while the rotary head assembly makes two complete rotations. The normal play digital video signals are recorded in two tracks substantially simultaneously by one head subassembly while the rotary head assembly makes only one complete rotation. Finally, the HDTV digital video signals are recorded in 4 tracks by both head subassemblies (one subassembly at a time) while the rotary head assembly makes one complete rotation. To achieve this, the magnetic tape on which these tracks are recorded are transported at different speeds during the extended play, normal play and HDTV modes, these different speeds exhibiting the ratio 1:2:4.
In the aforedescribed apparatus, since the signals that are recorded are processed to exhibit the same data rate for t he extended play, normal play and HDTV modes, even though the original transmission rates of these signals differ from each other, separate recording electronics for each mode need not be provided. This permits the recording circuitry to be relatively simplified; and the overall reliability of the apparatus is improved. Furthermore, since the rotational speed of the rotary head assembly is constant and need not be changed from one mode to another, the mechanical elements and construction of the assembly likewise may be simplified, thus providing better overall reliability. Moreover, the video signals which are recorded on the magnetic medium are recorded with a substantially constant recording density and, therefore, the recording medium may be used more effectively.
To increase the number of tracks recorded on the magnetic medium, it is desirable to reduce the width of each track which, or course, increases the recording density and, thus, increases the amount of information that may be recorded on a magnetic tape of given length. However, it has been believed heretofore that, if the track width is reduced, special tracking systems must be employed to insure accurate tracking of the individual heads during a playback mode. Thus, it had been thought that ATF, DTF (with a pilot signal) and other tracking techniques must be used. Consistent with this belief that special tracking systems must be provided, it also had been thought that the linearity of each track being recorded must be strictly maintained. This, in turn, requires the rotary head assembly and drive mechanism to be manufactured with limited tolerances and high accuracies. However, the requirement of special tracking control circuits and precise rotary mechanisms results in high manufacturing costs and, in addition, presents a serious risk that the reliability and operating stability of the overall apparatus may face substantial degradation.